Optical waveguide and method for fabricating the same

ABSTRACT

An optical waveguide comprising cladding  1  and core segment  20  buried in cladding  1  and serving as a waveguide, wherein a combination of glass material constituting the core segment  20  and another glass material constituting the cladding  1  is so selected that an absolute value of difference in coefficient of thermal expansion between these materials (α1−α2) is within a range of 0 and 9×10 −7 /° C., where α1 denotes a coefficient of thermal expansion of the former material and α2 denotes that of the latter material. Since this makes possible to bond directly the glass materials having different refraction factors and different coefficients of thermal expansion, and to produce the optical waveguide at even a lower temperature as compared to the prior art method as an upper cladding layer is formed with the sputtering method, it realizes reduction in number of processes and time of manufacture, thereby providing the optical waveguide of low transmission loss at low cost, as well as a method of manufacturing the same.

FIELD OF THE INVENTION

[0001] The present invention relates to an optical component, and inparticular, it relates to an optical waveguide and a method ofmanufacturing the same.

BACKGROUND OF THE INVENTION

[0002] In optical components of a waveguide system such as opticalsplitters, optical combiners, optical dividers, optical multiplexers andthe like, there hitherto are numerous methods of forming opticalwaveguides on a substrate, which can be categorized generally into twogroups that are deposition method and ion exchange method.

[0003] Deposition method is the method of forming an SiO₂ film on asubstrate made of silicon and the like, and it includes the flamehyrolysis deposition method, the plasma assisted chemical vapordeposition method (P-CVD method), the molecular beam epitaxy method (MBEmethod), and the like, to be more specific.

[0004] Referring now to FIG. 3, description is provided hereinafter of amethod of manufacturing an optical waveguide using the flame hyrolysisdeposition method representing those of the above deposition methods.After fine-grained silicon dioxide (SiO₂) added with germanium dioxide(GeO₂), or fine-grained silicon dioxide (SiO₂) added with titanium oxide(TiO₂) is deposited into a thickness of several μm on quartz substrate7, it is sintered and transparentized to form core layer 8, as shown inFIG. 3(a). Next, the core layer 8 is patterned into a predeterminedshape by using the photolithography and dry etching techniques to formcore segments 81, as shown in FIG. 3(b). Following the above, silicondioxide (SiO₂) added with phosphorus pentaoxide (P₂O₅) and boric oxide(B₂O₃) is deposited over the core segments 81 and the quartz substrate 7again by the flame hyrolysis deposition method, and it istransparentized by sintering to form upper cladding layer 9, as shown inFIG. 3(c), to thus complete the optical waveguide.

[0005] On the other hand, described next is a manufacturing method usingthe ion exchange method. In this method, a borosilicate glass substrateis used, and a waveguide pattern is formed on this substrate by using ametal mask. This substrate is then immersed in molten salt containingdopant, to let the dopant diffuse into the glass within the waveguidepattern by taking advantage of the ion-exchange phenomenon of the dopantwith the glass components, and to form a core layer in a surface layerof the glass substrate. Because the ion exchange phenomenon does notoccur in an area covered by the metal mask, the core layer is producedonly in an exposed area into a shape identical to that of the maskpattern.

[0006] In addition, the glass substrate is immersed in another moltensalt containing only components other than the aforesaid dopant amongthose present in the glass substrate, and an electric field is appliedto the substrate to cause the dopant in the core layer formed in thesurface layer to migrate toward inside of the glass substrate, to formcore segments of a predetermined shape in the glass substrate. Thesecore segments are parts having a high refraction factor so as to becomeoptical waveguides. After removal of the metal mask, the surface layerof the glass substrate is clad with a material of low refraction factorin a manner to cover the core segments, which serve the opticalwaveguides, into such a configuration that the core segments are buried.

[0007] However, there exist the following problems in the depositionmethod and the ion exchange method described above.

[0008] Although the optical waveguide formed by the deposition methodshows the smallest transmission loss, it has a problem in productivitybecause it requires many processes to make the optical waveguide as itmust go through the deposition and sintering processes for the coresegments and the cladding layer, and the patterning process using thephotolithography method, each of which requires a long duration ofprocessing time. For instance, the core layer requires 1.5 hours fordeposition and 10 hours for sintering.

[0009] On the other hand, although the optical waveguide can be formedin a comparatively short time with the ion exchange method, it producesuneven distribution of the dopant in a direction of thickness of theglass substrate because the core layer is formed by ion exchange. Thisconsequently produces uneven variation of refraction factor in thedirection of thickness and increases the transmission loss, therebygiving rise to a problem.

SUMMERY OF THE INVENTION

[0010] The present invention addresses the aforesaid problems of theprior art, and it is an object of the invention to provide an opticalwaveguide of low transmission loss, and a method of manufacturing theoptical waveguide with low cost by reducing a number of processes aswell as time required for manufacturing the same.

[0011] To achieve the above object, the present invention includesstructures as described hereinafter.

[0012] An optical waveguide of this invention comprises a cladding and acore segment buried in the cladding and serving as a waveguide, whereina glass material constituting the core segment and another glassmaterial constituting the cladding are so combined that an absolutevalue of difference in coefficient of thermal expansion between thesematerials (α1−α2) is within a range of 0 and 9×10⁻⁷/° C., when α1denotes a coefficient of thermal expansion of the former material and α2denotes that of the latter material.

[0013] Since this makes possible to bond directly between the coresegment and the lower cladding layer by using glass materials havingdifferent refraction factors and different coefficients of thermalexpansion, it realizes good optical characteristic at their interfaceand increases bonding strength. In addition, the optical waveguide canbe produced at even a lower temperature as compared to the prior artmethod when the upper cladding layer is formed with the sputteringmethod, thereby realizing not only improvement of flexibility inselecting the materials but also reduction of the cost.

[0014] Furthermore, the optical waveguide of this invention comprises acladding and a core segment buried in the cladding and serving as awaveguide, wherein a glass material constituting the core segment andanother glass material constituting the cladding are so combined that anabsolute value of difference in coefficient of thermal expansion betweenthese materials (α1−α2) is within a range of 0 and 9×10⁻⁷/° C., and ayield point of the glass material constituting the core segment ishigher than that of the glass material constituting the cladding by 70°C. or greater, when α1 and At1 denote a coefficient of thermal expansionand the yield point of the former material, and α2 and At2 denote thoseof the latter material.

[0015] Because this makes possible not only to form the upper claddinglayer by the sputtering method, but also to adopt an alternative methodin which a glass plate constituting the cladding layer is integratedinto one body with a lower cladding layer by hot pressing in a manner tobury the core segment into the cladding layer, thereby realizing a costreduction by way of reducing a number and time of the manufacturingprocesses.

[0016] Also, the optical waveguide of this invention has a structure, inwhich the glass material constituting the core segment comprisesborosilicate crown glass having a composition of silicon dioxide (SiO₂),boric oxide (B₂O₃), alkaline metal oxide (R₂O) and diatomic metal oxide(LO) (R: alkaline metal and L: diatomic metal), and the glass materialconstituting the cladding comprises fluorine crown glass having acomposition of silicon dioxide (SiO₂), boric oxide (B₂O₃) and fluorinecompound. Because of this structure, the optical waveguide has anexcellent transmission loss factor, and it can be made with low cost.

[0017] Furthermore, the optical waveguide of this invention has anotherstructure, in which glass materials constituting both the core segmentand the cladding comprise fluorine crown glass having a composition ofsilicon dioxide (SiO₂), boric oxide (B₂O₃) and fluorine compound, andthe materials are so combined that a refraction factor of the coresegment is greater than that of the cladding. Because of this structure,the optical waveguide has an excellent transmission loss factor, and itcan be made with low cost.

[0018] Also, a method of manufacturing the optical waveguide accordingto this invention comprises a step of forming a bonded substrate bypressing and heating a glass plate constituting a lower cladding layerand another glass plate constituting a core layer, which are arranged ina manner that optically polished surfaces of them abut against eachother to make direct bonding, a step of finishing a surface of the corelayer of the bonded substrate by grinding and/or polishing to obtain athickness appropriate for a core segment, a step of forming the coresegment defining a waveguide by etching the core layer, and a step offorming an upper cladding layer by making a film with sputtering overthe lower cladding layer, including the core segment, while burying thecore segment at the same time into a cladding comprised of the uppercladding layer and the lower cladding layer. Accordingly, the method canproduce the optical waveguide composed of glass materials quite easilywhile ensuring excellent characteristics.

[0019] Moreover, another method of manufacturing the optical waveguideaccording to this invention comprises a step of forming a bondedsubstrate by pressing and heating a glass plate constituting a lowercladding layer and another glass plate constituting a core layer, whichare arranged in a manner that optically polished surfaces of them abutagainst each other to make direct bonding, a step of finishing a surfaceof the core layer of the bonded substrate by grinding and/or polishingto obtain an appropriate thickness for core segment, a step of formingthe core segment defining a waveguide by etching the core layer, and astep of arranging on the core segment an upper cladding layer of glassplate having a yield point lower than that of the core segment, andhot-pressing them by means of heating and pressing at a temperature atleast equal to or higher than the yield point of the upper claddinglayer, to bond together the upper cladding layer and the lower claddinglayer into an integral body and to bury the core segment at the sametime into the cladding while retaining an original shape of the coresegment. Accordingly, this method can produce the optical waveguide ofexcellent optical characteristics in a short time with simplemanufacturing processes.

[0020] Furthermore, the method of manufacturing the optical waveguide ofthis invention employs a glass material constituting the core segmentand another glass material constituting the cladding, wherein anabsolute value of difference in coefficient of thermal expansion betweenthese materials (α1−α2) is within a range of 0 and 9×10⁻⁷/° C., when α1denotes a coefficient of thermal expansion of the former material and α2denotes that of the latter material. The method can produce easily theoptical waveguide which is not liable to produce any crack or separationeven after the processes of direct bonding, dicing and the like.

[0021] Moreover, the method of manufacturing the optical waveguide ofthis invention employs a glass material constituting the core segmentand another glass material constituting the cladding, wherein anabsolute value of difference in coefficient of thermal expansion betweenthese materials (α1−α2) is within a range of 0 and 9×10⁻⁷/° C., and ayield point of the glass material constituting the core segment ishigher than that of the glass material constituting the cladding by 70°C. or greater, when α1 and At1 denote a coefficient of thermal expansionand the yield point of the former material, and α2 and At2 denote thoseof the latter material. This method can produce optical waveguides withconstant precision in shape of the core segments and good productivityin the mass-production even though the upper cladding layers arehot-pressed.

[0022] The method of manufacturing the optical waveguide of thisinvention includes the step of hot pressing using a temperature higherthan the yield point of the glass material constituting the cladding butlower than the yield point of the glass material constituting the coresegment. By setting the hot pressing temperature within the above range,this method can reliably bond and integrate the upper cladding layer andthe lower cladding layer into one body while controlling deformation ofthe core segment.

[0023] Moreover, the method of manufacturing the optical waveguide ofthis invention employs glass material constituting the core segment,which comprises borosilicate crown glass containing silicon dioxide(SiO₂), boric oxide (B₂O₃), alkaline metal oxide (R₂O) and diatomicmetal oxide (LO) (R: alkaline metal and L: diatomic metal), and glassmaterial constituting the cladding, which comprises fluorine crown glasscontaining silicon dioxide (SiO₂), boric oxide (B₂O₃) and fluorinecompound. This method can make the direct bonding easy so as to increasebonding strength at the interface, and produce highly reliable opticalwaveguide.

[0024] Also, the method of manufacturing the optical waveguide of thisinvention employs glass materials constituting both the core segment andthe cladding, which comprise fluorine crown glass containing silicondioxide (SiO₂), boric oxide (B₂O₃) and fluorine compound, and acombination of these materials are so selected that a refraction factorof the core segment is greater than that of the cladding. This methodcan also make the direct bonding easy so as to increase bonding strengthat the interface, and produce highly reliable optical waveguide.

[0025] Moreover, the method of manufacturing the optical waveguide ofthis invention includes the step of direct bonding only after bothbonding surfaces of the lower cladding layer and the core layer arepolished to 0.1 nm to 1 nm in arithmetic mean surface roughness (Ra) and0.1 μm to 1 μm in flatness throughout the entire surface areas of thesubstrate being bonded. This method can thus ensure the direct bondingover the entire surfaces even though it uses a substrate of 3 inches orlarger in diameter, for instance, so as to allow manufacturing ofoptical waveguides with high productivity. In this method, 1 nm or lessin the surface roughness Ra can establish reliable interatomic bondingbetween glass components of the substrate, and 1 □m or less in flatnesscan make uniform contact of the substrate over the entire surface areas,thereby ensuring reliability and consistency in the direct bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 illustrates processing diagrams showing an opticalwaveguide and a method of manufacturing the same according to a firstexemplary embodiment of the present invention;

[0027]FIG. 2 illustrates processing diagrams showing another opticalwaveguide and a method of manufacturing the same according to a secondexemplary embodiment of the present invention; and

[0028]FIG. 3 illustrates processing diagrams showing a method ofmanufacturing an optical waveguide using the flame deposition hydrolysismethod of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Referring now to the accompanying drawings, detailed descriptionswill be provided hereinafter of optical waveguides and methods ofmanufacturing the same according to the preferred embodiments of thepresent invention.

[0030] (First Exemplary Embodiment)

[0031]FIG. 1 illustrates processing diagrams for manufacturing anoptical waveguide according to the first exemplary embodiment of thepresent invention. A circular shape fluorine crown glass of 3 inches indiameter, as a substrate constituting lower cladding layer 1 a, and aborosilicate crown glass of the same shape, as another substrateconstituting core layer 2 were directly bonded together. FIG. 1(a) showsan appearance of the bonded state. The direct bonding is made in thefollowing manner. First, one surface of each of the glass substrates isoptically polished to such an extent that it becomes 1 nm in arithmeticmean surface roughness (Ra) and 1 μm in flatness. After that, theseglass substrates are rinsed to a level of cleanliness that a contactangle of water to these glass substrates is 5 degrees or less. Next,after the polished surfaces of these glass substrates are abutted andpressed against each other, they are heat-treated at 250° C. for onehour to bond the lower cladding layer 1 a and the core layer 2 directlyin the atomic level, and to form integrated bonded substrate 11. In thedirect bonding, when hyaline surfaces of oxide such as glass substratesare normalized in the atomic level and heat-treated while being abuttedand pressed against each other, there is produced bonding of atoms ofthe glass substrate components via oxygen atoms, to gain an interface asstrong as a solid bulk. According to the direct bonding as described,integration of materials can be made easily without using any adhesiveagent even when they are large in diametral size and different materialscontaining different compositions.

[0032] Next, a surface of the core layer 2 is ground and polished to afinal thickness of 5 to 7 μm, as shown in FIG. 1(b). The core layer 2 isthen coated with photoresist 3 by the spin-coating method, as shown inFIG. 1(c). Afterwards, the photoresist 3 is patterned by thephotolithography method as shown in FIG. 1(d), and the unnecessaryportion of the core layer 2 is removed by dry etching as shown in FIG.1(e). This obtains substrate 11 a provided with core segments 20 of apredetermined shape formed on the lower cladding layer 1 a, when thephotoresist 3 is removed thereafter.

[0033] Following the above, upper cladding layer 1 b is formed bydepositing fluorine crown glass with the sputtering method on thesurface where the core segments 20 are formed as shown in FIG. 1(f)using as a target the same fluorine crown glass as the lower claddinglayer 1 a. Since the lower cladding layer 1 a and the upper claddinglayer 1 b are formed of the same material, they are integrated into onebody to form cladding 1, so as to obtain the optical waveguide in whichcore segments 20 are buried into the cladding 1. This is shown in FIG.1(g).

[0034] Concreately embodied experiment sample is described hereinafterwith reference to a comparison sample.

[0035] For a first experiment sample, lower cladding layer 1 a wasformed using fluorine crown glass having a glass yield point of 568° C.,a refraction factor of 1.4876, a coefficient of thermal expansion of95×10⁻⁷/° C., and composed of SiO₂, B₂O₃, K₂O and KHF₂. Also, core layer2 was formed using borosilicate crown glass having a glass yield pointof 625° C., a refraction factor of 1.5164, a coefficient of thermalexpansion of 86×10⁻⁷/° C., and composed of SiO₂, B₂O₃, Na₂O, K₂O andBaO. These glass substrates are circular shape, and each has 1 mm inthickness and 3 inches in diameter.

[0036] After each of surfaces to be abutted was optically polished to asurface roughness (Ra) of 1 nm and flatness of 1 μm, and rinsed to suchcleanliness that a contact angle of water to the glass substrate becomes5 degrees or less, the polished surfaces were abutted and pressed uponeach other, and subjected to a heat treatment at 250° C. for 1 hour tobond them directly. The borosilicate crown glass constitutong core layer2 was then ground and polished until it becomes 7 μm thick. Next,photoresist 3 was formed on the borosilicate crown glass, and it waspatterned by exposing it to light through a mask pattern placed thereonand by developing it thereafter. Using the patterned photoresist 3 as amask, the core layer 2 was subjected to the reactive ion etching, toform core segments 20 of 7 μm square in cross-sectional shape.Subsequently, fluorine crown glass of the same composition as the lowercladding layer 1 a was sputtered to a thickness of 20 μm, which formedupper cladding layer 1 b, and thus completed the optical waveguides.Both the lower cladding layer 1 a and the upper cladding layer 1 b weremade of the same material having the same optical characteristics, andthey composed integrated cladding 1. The sample produced as above isdesignated first experiment sample.

[0037] The optical waveguides of this first experiment sample has anabsolute value of 9×10⁻⁷/° C. in difference of coefficient of thermalexpansion between the core segments 20 and the cladding 1 (α1−α2).However, the sample have a substantially great bonding strength in thedirectly bonded interface between the core segments 20 and the cladding1, that it did not show any sign of separation at all. In addition,since the lower cladding layer 1 a and the core segments 20 are coveredsufficiently with the upper cladding layer 1 b formed by the sputtering,there were not observed any air bubbles which are liable to be trappedin any of the interfaces between the core segments 20 and the lowercladding layer 1 a, and between the core segments 20 and the uppercladding layer 1 b.

[0038] In addition, it is necessary that a plurality of the opticalwaveguides produced in the circular shape glass substrate of 3-inchdiameter are cut into individual segments, and ends of the core segments20 are also cut by a dicer or the like to expose their surfaces in orderto connect them with optical fibers. If there is a large difference inthe coefficient of thermal expansion between the core segments 20 andthe cladding 1, it is likely that a break or a crack can occur in thecore segments 20, cladding 1, and especially around their interfacialareas during this process. However, such a defect did not occur in theoptical waveguides of the first experiment sample.

[0039] As a second experiment sample, circular shape fluorine crownglass substrates having 3 inches in diameter and 1 mm in thickness wereused for core layer and lower cladding layer 1 a. However, the fluorinecrown glass used for the lower cladding layer 1 a has a glass yieldpoint of 491° C., a refraction factor of 1.4816, a coefficient ofthermal expansion of 95×10⁷/° C., and composed of SiO₂, B₂O₃, AlF₃, K₂Oand Na₂O. Also, the fluorine crown glass used for the core layer 2 has aglass yield point of 568° C., a refraction factor of 1.4876, acoefficient of thermal expansion of 95×10⁻⁷/° C., and composed of SiO₂,B₂O₃, K₂O and KF. The optical waveguides were produced using these glasssubstrates in the same method and conditions as those of the firstexperiment sample. The sample produced in the manner as described aboveis designated second experiment sample.

[0040] There was not any warp and separation at the directly bondedinterface, nor were any air bubbles trapped around the same interfacebetween the core segments 20 and the cladding 1 in the opticalwaveguides of the second experiment sample. In addition, there was notany break or crack occured during the process of dicing.

[0041] For the purpose of comparison with the above experiment samples,fluorine crown glass of 3 inches in diameter and 1 mm in thickness wasused as lower cladding layer 1 a, and borosilicate crown glass of thesame shape as core layer 2. The fluorine crown glass used for the lowercladding layer 1 a has a glass yield point of 495° C., a refractionfactor of 1.5112, a coefficient of thermal expansion of 100×10⁻⁷/° C.,and composed of SiO₂, KF, and K₂O. Also, the borosilicate crown glassused for the core layer 2 has a glass yield point of 625° C., arefraction factor of 1.5164, a coefficient of thermal expansion of86×10⁻⁷/° C., and composed of SiO₂, B₂O₃, Na₂O, K₂O and BaO. Opticalwaveguides were produced using these glass substrates in the same methodand conditions as those of the first experiment sample. The opticalwaveguides produced as above is designated first comparison sample.

[0042] In the case of first comparison sample, these optical waveguidesshowed an absolute value of 14×10⁻⁷/° C. in difference of coefficient ofthermal expansion between the core segments 20 and the cladding 1(α1−α2).

[0043] During the process of direct bonding for these opticalwaveguides, there was not observed any warp in the substrate, noseparation at the interface between the core layer 2 and the lowercladding layer 1 a, nor were air bubbles trapped around the sameinterface between the core segments 20 and the cladding 1, as were thecases of the first experiment sample and the second experiment sample.However, there was a crack occurred during dicing in part of theinterfaces between the core segments 20 and cladding 1.

[0044] Table 1 shows results of these three different samples. TABLE 11st 2nd Experiment Experiment 1st Comparison Sample Sample Sample Yieldpoint Cladding 568 491 495 (° C.) Core 625 568 625 segments CoefficientCladding 95 95 100 of thermal Core 86 95 86 expansion segments (×10⁻⁷/°C.) Refraction Cladding 1.4876 1.4816 1.5112 factor Core 1.5164 1.48761.5164 segments α1-α2 (×10⁼⁷/° C.) 9 0 14 Difference in Yield 57 77 130point (° C.)

[0045] As is obvious from Table 1, the value (α1−α2) of the firstexperiment sample is 9×10⁻⁷/° C., the value (α1−α2) of the secondexperiment sample is 0, and the value (α1−α2) of the first comparisonsample is 14×10⁻⁷/° C. On the other hand, the first comparison samplewas the only one in which a crack was observed after it was subjected tothe processes of direct bonding, dicing, and the like. In addition,Optical waveguides of like shape were produced by using a variety ofmaterials having different coefficients of thermal expansion accordingto the same manufacturing method as described above, and they wereexamined for cracks, separations, and presence of air bubbles. As aresult, cracks were found occurred during dicing at least on thoseoptical waveguides that have differences (α1−α2) of the coefficient ofthermal expansion equal to or greater than 10×10⁻⁷/° C. However, noabnormality of crack or the like was observed on those having values of9×10⁻⁷/° C. or less.

[0046] According to the above results, it was found that the differenceof coefficient of thermal expansion between the core layer 2 and thecladding 1 need to be within a range of 0 and 9×10⁻⁷/° C. in order tomanufacture the optical waveguides by making direct bonding betweenlower cladding layer 1 a and core layer 2 and forming upper claddinglayer 1 b by sputtering.

[0047] (Second Exemplary Embodiment)

[0048]FIG. 2 illustrates processing diagrams for manufacturing anoptical waveguide according to the second exemplary embodiment of thepresent invention. A circular shape fluorine crown glass of 3 inches inthe diameter, as a substrate constituting lower cladding layer 4 a, andanother fluorine crown glass of the same shape, as a substrateconstituting core layer 5 were put together by direct bonding. FIG. 2(a)shows an appearance of the bonded state. The method of making the directbonding is same as that described in the first exemplary embodiment, anddetails are therefore skipped. Bonding directly the core layer 5 and thelower cladding layer 4 a obtains bonded substrate 21 in which the corelayer 5 and the lower cladding layer 4 a are integrated into one body.

[0049] Next, a surface of the core layer 5 is ground and polished untila thickness of the core layer 5 becomes 5 to 7 μm, as shown in FIG.2(b). The core layer 5 is then coated with photoresist 6 by thespin-coating method, as shown in FIG. 2(c). Afterwards, the photoresist6 is patterned by the photolithography method as shown in FIG. 2(d), andthe unnecessary portion of the core layer 5 is removed by dry etching.This is shown in FIG. 2(e). Removal of the photoresist 6 now obtainssubstrate 21 a provided with core segments 50 formed thereon. This isshown in FIG. 2(f).

[0050] Prepared thereafter is upper cladding layer 4 b made of fluorinecrown glass, which is same material as the lower cladding layer 4 a.After the upper cladding layer 4 b is placed on top of the core segments50 as shown in FIG. 2(g), it is hot pressed at a temperature higher thanthe glass yield point of the upper cladding layer 4 b by 20 to 30° C. tobury the core segments 50 and to form cladding 4 by integrating thelower cladding layer 4 a and the upper cladding layer 4 b into one body.

[0051] For this hot pressing, it is necessary that a combination of theglass materials is so chosen as to satisfy the formula of At1−At2>70,where At1 denotes a yield point of the glass material constituting thecore layer 5 and At2 denotes that of the glass material constituting thecladding. With this combination, it is possible to set a range of theheating temperature that can avoid the core segments 50 from warpingduring the hot pressing, and facilitate integration of the uppercladding layer and the lower cladding layer.

[0052] Concreately embodied experiment samples are described hereinafterwith reference to comparison samples.

[0053] For a third experiment sample, fluorine crown glasses of 3 inchesin diameter and 1 mm in thichmess were used as the core layer 5 and thelower cladding layer 4 a, after one side surface of each of them wasoptically polished and rinsed. However, the fluorine crown glass usedfor the lower cladding layer 4 a has a yield point (At2) of 491° C., arefraction factor of 1.4816, a coefficient of thermal expansion (α2) of95×10⁻⁷/° C., and composed of SiO₂, B₂O₃, AlF₃, K₂O and Na₂O. Also, thefluorine crown glass used for the core layer 5 has a yield point (At1)of 568° C., a refraction factor of 1.4876, a coefficient of thermalexpansion (α1) of 95×10⁻⁷/° C., and composed of SiO₂, B₂O₃, K₂O and KF.The polished surface of the lower cladding layer 4 a and the polishedsurface of the core layer 5 were abutted and pressed upon each other,and subjected to a heat treatment at 250° C. for 1 hour to bond themdirectly. Roughness (Ra) and flatness of the surfaces after opticallypolished, and a contact angle of water to the substrate after rinsingwere in the same degrees as those of the first exemplary embodiment.

[0054] The core layer 5 was then ground and polished until the thicknessbecames 7 μm. Following the above, the core layer 5 was coated withphotoresist 6, and it was exposed to light in a prescribed manner topattern-form the photoresist 6. Using the patterned photoresist 6 as amask, the core layer 5 was dry-etched to form core segments 50 ofapproximately 7 μm square in cross-sectional shape.

[0055] Subsequently, the upper cladding layer 4 b composed of the sameglass material as the lower cladding layer 4 a was placed on top of thecore segments 50, and it was hot-pressed at 520° C. for 30 seconds. Thehot press causes the upper cladding layer 4 b to deform in a manner tobury the core segments 50 into it until it comes into contact with thelower cladding layer 4 a, and to form the optical waveguides when theupper cladding layer 4 b and the lower cladding layer 4 a are integratedto become cladding 4. The sample thus produced is designated thirdexperiment sample.

[0056] The optical waveguides of the third experiment sample did notcause deformation of the core segments 50, and retained an excellentprecision in shape even after they were subjected to the hot pressing.Moreover, there was not any warp of the substrate or separation at thebonded interface, nor were any air bubbles trapped around the sameinterface between the core segments 50 and the cladding even after theprocesses of direct bonding, subsequent hot-pressing, and the like.

[0057] For a fourth experiment sample, fluorine crown glass of 3 inchesin diameter and 1 mm in thickness was used as the lower cladding layer 4a, after one side surface was optically polished and rinsed. Also,borosilicate crown glass of the same shape and rinsed in the same waywas used as the core layer 5. However, the fluorine crown glass used forthe lower cladding layer 4a has a yield point (At2) of 447° C., arefraction factor of 1.4644, a coefficient of thermal expansion (α2) of93×10⁻⁷/° C., and composed of SiO₂, B₂O₃, K₂O and KHF₂. Also, theborosilicate crown glass used for the core layer 5 has a yield point(At1) of 625° C., refraction factor of 1.5164, a coefficient of thermalexpansion (α1) of 86×10⁻⁷/° C., and composed of SiO₂, B₂O₃, Na₂O, K₂Oand BaO. Although the optical waveguides were produced with these glassmaterials, same conditions were used as those of the third experimentsample except that the hot press was given at 470° C. The sample thusproduced is designated fourth experiment sample.

[0058] The optical waveguides of the fourth experiment sample did notcause warp and separation, nor were any air bubbles trapped around theinterface between the core segments 50 and the cladding 4 even after theprocesses of direct bonding, subsequent hot-pressing and the like.Moreover, there was not any break, crack or the like due to the dicing.Furthermore, the core segments 50 did not deform by the heat after beingsubjected to the hot press at 470° C., and retained an excellentprecision in shape.

[0059] Next, samples of optical waveguide were produced as secondcomparison sample for the purpose of comparison with the aboveexperiment samples. Fluorine crown glass of 3 inches in diameter and 1mm in thichmess was used for the lower cladding layer 4 a, after oneside surface was optically polished and rinsed. Also, borosilicate crownglass of the same shape and processed in the same way was used for thecore layer 5. However, the fluorine crown glass used for the lowercladding layer 4 a has a yield point (At2) of 568° C., a refractionfactor of 1.4876, a coefficient of thermal expansion (α2) of 95×10⁻⁷/°C., and composed of SiO₂, B₂O₃, K₂O and KHF₂. Also, the borosilicatecrown glass used for the core layer 5 has a yield point (At1) of 625°C., refraction factor of 1.5164, a coefficient of thermal expansion (α1)of 86×10⁻⁷/° C., and composed of SiO₂, B₂O₃, Na₂O, K₂O and BaO. Althoughthe optical waveguides were produced with these glass materials, sameconditions were used as those of the third experiment sample except thatthe hot press was given at 590° C.

[0060] The optical waveguides produced as above did not cause warp ofthe substrate and separation in the interface, nor was any break, crackor the like due to the dicing even after the manufacturing processes ofdirect bonding, subsequent hot-pressing and the like. Moreover, the coresegments 50 did not deform by the heat after being subjected to the hotpress, and retained an excellent precision in shape. However, there wereair bubbles trapped around the interfaces between the core segments 50and the upper cladding layer 4 b as well as between the lower uppercladding layer 4 a and the upper cladding layer 4 b, indicatingdeficiency in integration of the cladding by the hot press.

[0061] Next, as a third comparison sample, fluorine crown glass of 3inches in diameter and 1 mm in thichmess having one side surfaceoptically polished and rinsed was used as the lower cladding layer 4 a,and borosilicate crown glass of the same shape and processed in the sameway was used as the core layer 5. The fluorine crown glass used for thelower cladding layer 4 a has a yield point (At2) of 495° C., arefraction factor of 1.5112, coefficient of thermal expansion (α2) of100×10⁻⁷/° C., and composed of SiO₂, KF and K₂O. Also, the borosilicatecrown glass used for the core layer 5 has a yield point (At1) of 625°C., refraction factor of 1.5164, coefficient of thermal expansion (α1)of 86×10⁻⁷/° C., and composed of SiO₂, B₂O₃, Na₂O, K₂O and BaO. Althoughthe optical waveguides were produced with these glass materials, samemanufacturing conditions were used as those of the third experimentsample. The sample thus produced is designated third comparison sample.

[0062] The optical waveguide produced as above showed no thermaldeformation on the core segments 50 even after they were hot-pressed,and retained an excellent precision in shape. Also, it did not show anywarp and separation of the substrate even after the processes of directbonding, hot-pressing and the like. In addition, no air bubbles wereobserved around the interfaces between the core segments 50 and theupper cladding layer 4 b, or between the upper cladding layer 4 b andthe lower upper cladding layer 4 a, as have occurred in the secondcomparison sample due to deficiency in the cladding integration by thehot press. However, there were partial cracks found occured in theinterface between the core segments 50 and the cladding 1 during thedicing.

[0063] Table 2 shows results of these four different samples. TABLE 2Third Fourth Second Third Experiment Experiment Comparison ComparisonSample Sample Sample Sample Yield Cladding 491 447 568 495 point (° C.)Core 568 625 625 625 segments Coefficient Cladding 95 93 95 100 ofthermal Core segments 95 86 86 86 expansion (×10⁻⁷/° C.) RefractionCladding 1.4816 1.4644 1.4876 1.5112 factor Core 1.4876 1.5164 1.51641.5164 segments α1-α2 (×10⁻⁷/° C.) 0 7 9 14 Difference in Yield 77 17857 130 point (° C.)

[0064] As is obvious from Table 2, the third experiment sample and thefourth experiment sample were the only samples that exhibited goodprecision in shape and did not cause any crack, separation or the like.These samples have a difference (α1−α2) of 7×10⁻⁷/° C. or less in thecoefficient of thermal expansion and a difference of 77° C. or higher inthe yield point. On the other hand, the second comparison sample has adifference in the yield point of only 57° C., while a difference in thecoefficient of thermal expansion (α1−α2) is 9×10⁻⁷/° C. Further,although the third comparison sample has a large difference of 130° C.in the yield point, it also has a large difference (α1−α2) of 14×10⁻⁷/°C. in the coefficient of thermal expansion. These differences in thecoefficient of thermal expansion and the yield point were examined oftheir conditions as to how they affect to preciseness of the shape,cracks and the like.

[0065] In the second comparison sample, although there was not any warpof the substrate and separation in the interface, nor was any crack dueto dicing even after the manufacturing processes of direct bonding,hot-pressing and the like, there were air bubbles trapped around theinterfaces between the core segments 50 and the upper cladding layer 4b. Presence of the air bubbles is ascribed to deficiency of the hotpress. Therefore, cladding tightness can be inproved and the air bubblesprevented to develop if the hot-press temperature is raised. On theother hand, raising the temperature substantially impairs preciseness offorming shape of the core segments 50.

[0066] In light of the difference in yield point for possibility ofimproving the cladding tightness with the fot press while ensuring alevel of preciseness in the shape, another examination was made for acondition of air-bubble trapping by using combinations of glassmaterials having a variety of different yield points. It was found as aresult that a difference of 70° C. or higher in temperature of the yieldpoint is needed between the upper cladding layer 4 b and the coresegments 50. In other words, it was determined that deformation on thecore segments 50 can be avoided and excellent bonding tightness isensured in their interface if there is a difference of 70° C. or higherin the temperature, since the hot-press temperature can be set to avalue higher than the yield point of the upper cladding layer 4 b by 20to 30° C. but lower than the yield point of the core segments 50. Thatis, in this manufacturing method for directly bonding the core layer 5and the lower cladding layer 4 a, and integrating the upper claddinglayer 4 b into one body with the lower cladding layer 4 a by the hotpress in a manner to bury the core segments 50 therein, it is necessaryto arrange an absolute value of difference in the coefficient of thermalexpansion (α1−α2) in a range of 0 to 9×10⁻⁷/° C. and a difference of 70°C. or higher in the yield point.

[0067] In these exemplary embodiments, although the descriptions wereprovided of the cases in which sodium (Na) and potassium (K) are givenas examples of the alkaline metal for the borosilicate crown glass,lithium (Li) may also be used. In addition, although the abovedescriptions were provided of the case of barium (Ba) as the diatomicmetal, it may be any of magnesium (Mg), calcium (Ca) and strontium (Sr).Moreover, although the lower cladding layer and the upper cladding layerwere composed by using the same material in these exemplary embodiments,they can be of different compositions so long as they are fluorine crownglasses of the same kind. Furthermore, although the above descriptionswere provided of the cases in which fluorine compound of KHF₃ and KFwere used as a component of the fluorine crown glasses, this inventionis not restrictive to these materials, and they may be substituted withany of MgF₂, CaF₂, SrF₂, BaF₂, LiF, NaF, and the like.

Industrial Applicability

[0068] As has been described, an optical waveguide and a method ofmanufacturing the same according to the present invention relates to astructure in which glass substrates are bonded directly in theinteratomic level, and core segments are buried into a cladding when anupper cladding layer is formed by means of sputtering or hot pressing.The invented method provides advantages of reducing a number of themanufacturing processes, shortening the processing time, andmanufacturing easily the optical wave guide having excellentcharacteristics.

What is claimed is:
 1. An optical waveguide comprising: a cladding; anda core segment buried in said cladding, said core segment serving as awaveguide, wherein a glass material constituting said core segment andanother glass material constituting said cladding are combined so thatan absolute value of difference in coefficient of thermal expansionbetween said glass materials (α1−α2) is within a range of 0 and 9×10⁻⁷/°C,. where α1 denotes a coefficient of thermal expansion of said glassmaterial and α2 denotes a coefficient of thermal expansion of saidanother glass material.
 2. An optical waveguide comprising: a cladding;and a core segment buried in said cladding, said core segment serving asa waveguide, wherein a glass material constituting said core segment andanother glass material constituting said cladding are combined so thatan absolute value of difference in coefficient of thermal expansionbetween said glass materials (α1−α2) is within a range of 0 and 9×10⁻⁷/°C., and further wherein a yield point of said glass materialconstituting said core segment is higher than a yield point of saidanother glass material constituting said cladding by 70° C. or greater,where α1 and At1 denote a coefficient of thermal expansion and the yieldpoint of said glass material, and α2 and At2 denote a coefficient ofthermal expansion and the yield point of said another glass material. 3.The optical waveguide according to any of claim 1 and claim 2, whereinsaid glass material constituting said core segment comprisesborosilicate crown glass having a composition of silicon dioxide (SiO₂),boric oxide (B₂O₃), alkaline metal oxide (R₂O) and diatomic metal oxide(LO) (R: alkaline metal and L: diatomic metal), and said another glassmaterial constituting said cladding comprises fluorine crown glasshaving a composition of silicon dioxide (SiO₂), boric oxide (B₂O₃) andfluorine compound.
 4. The optical waveguide according to any of claim 1and claim 2, wherein said glass materials constituting both said coresegment and said cladding comprise fluorine crown glass having acomposition of silicon dioxide (SiO₂), boric oxide (B₂O₃) and fluorinecompound, and a combination of said glass materials is so arranged thata refraction factor of said core segment is greater than a refractionfactor of said cladding.
 5. A method of manufacturing optical waveguidecomprising the steps of: forming a bonded substrate by pressing andheating a glass plate constituting a lower cladding layer and anotherglass plate constituting a core layer arranged in a manner thatoptically polished surfaces thereof abut against each other, to makedirect bonding; finishing a surface of said core layer of said bondedsubstrate by one of grinding and polishing, to obtain a thicknessappropriate for a core segment; forming said core segment defining awaveguide by etching said core layer; and forming an upper claddinglayer by making a film with sputtering over said lower cladding layer,including said core segment, while burying said core segment at the sametime into a cladding comprised of said upper cladding layer and saidlower cladding layer.
 6. A method of manufacturing optical waveguidecomprising the steps of: forming a bonded substrate by pressing andheating a glass plate constituting a lower cladding layer and anotherglass plate constituting a core layer arranged in a manner thatoptically polished surfaces thereof abut against each other, to makedirect bonding; finishing a surface of said core layer of said bondedsubstrate by one of grinding and polishing, to obtain a thicknessappropriate for a core segment; forming said core segment defining awaveguide by etching said core layer; and arranging on said core segmentan upper cladding layer of glass plate having a yield point lower than ayield point of said core segment, and hot-pressing said upper claddinglayer by means of heating and pressing at a temperature at least equalto or higher than the yield point of said upper cladding layer, to bondtogether said upper cladding layer and said lower cladding layer into anintegral body and to bury said core segment at the same time into acladding while retaining an original shape of said core segment.
 7. Themethod of manufacturing optical waveguide according to one of claim 5and claim 6, said method employing a glass material constituting saidcore segment and another glass material constituting said cladding,wherein an absolute value of difference in coefficient of thermalexpansion (α1−α2) between said glass materials is within a range of 0and 9×10⁻⁷/° C., where α1 denotes a coefficient of thermal expansion ofsaid glass material and α2 denotes a coefficient of thermal expansion ofsaid another glass material.
 8. The method of manufacturing opticalwaveguide according to claim 6, said method employing a glass materialconstituting said core segment and another glass material constitutingsaid cladding, wherein an absolute value of difference in coefficient ofthermal expansion between said glass materials (α1−α2) is within a rangeof 0 and 9×10⁻⁷/° C., and a yield point of said glass materialconstituting said core segment is higher than a yield point of saidglass material constituting said cladding by 70° C. or higher, where α1and At1 denote a coefficient of thermal expansion and the yield point ofsaid glass material, and α2 and At2 denote a coefficient of thermalexpansion and the yield point of said another glass material.
 9. Themethod of manufacturing optical waveguide according to one of claim 6and claim 8, wherein said step of hot pressing includes heating at atemperature higher than the yield point of said glass materialconstituting said cladding but lower than the yield point of said glassmaterial constituting said core segment.
 10. The method of manufacturingoptical waveguide according to any one of claim 7 through claim 9,wherein said glass material constituting said core segment comprisesborosilicate crown glass containing silicon dioxide (SiO₂), boric oxide(B₂O₃), alkaline metal oxide (R₂O) and diatomic metal oxide (LO) (R:alkaline metal and L: diatomic metal), and said another glass materialconstituting said cladding comprises fluorine crown glass containingsilicon dioxide (SiO₂), boric oxide (B₂O₃) and fluorine compound. 11.The method of manufacturing optical waveguide according to any one ofclaim 7 through claim 9, wherein said glass materials constituting bothsaid core segment and said cladding comprise fluorine crown glasscontaining silicon dioxide (SiO₂), boric oxide (B₂O₃) and fluorinecompound, and a combination of said glass materials is so arranged thata refraction factor of said core segment is greater than a refractionfactor of said cladding.
 12. The method of manufacturing opticalwaveguide according to one of claim 5 and claim 6, wherein said directbonding is made only after both bonding surfaces of said lower claddinglayer and said core layer are polished to 0.1 nm to 1 nm in arithmeticmean surface roughness (Ra) and 0.1 μm to 1 μm in flatness throughoutthe entire surface areas of said substrate to be bonded.